Quite simply you cannot. It is widely stated you would need to be at a height of at least 40,000 ft to get even a hint of curvature if earth were round. Commercial aircraft are not allowed to fly this high. They are only allowed to fly just under this altitude. 36,000ft might be typical. In addition, the windows on commercial aircraft are small and heavily curved. Even if they flew high enough for a person to see curvature, it would still not be visible to passengers.

79.) The remark is common enough that we can see the circle of the Earth if we cross the ocean, and that this proves it to be round. Now, if we tie a donkey to a stake on a level common, and he eats the grass all around him, it is only a circular disc that he has to do with, not a spherical mass. Since, then, circular discs may be seen anywhere – as well from a balloon in the air as from the deck of a ship, or from the standpoint of the donkey, it is a proof that the surface of the Earth is a plane surface, and, therefore, a proof that the Earth is not a globe.

161) If Earth were really a ball, there would be no reason to use rockets for flying into “outer-space” anyway because simply flying an airplane straight at any altitude for long enough should and would send you off into outer-space. To prevent their airplanes from flying tangent to the ball-Earth, pilots would have to constantly course-correct downwards, or else within just a few hours the average commercial airliner traveling 500mph would find themselves lost in “outer-space.” The fact that this never happens, artificial horizons remain level at pilot’s desired altitudes and do NOT require constant downwards adjustments, proves the Earth is not a ball.
The remark is common enough that we can see the circle of the Earth if we cross the ocean, and that this proves it to be round. Now, if we tie a donkey to a stake on a level common, and he eats the grass all around him, it is only a circular disc that he has to do with, not a spherical mass. Since, then, circular discs may be seen anywhere - as well from a balloon in the air as from the deck of a ship, or from the standpoint of the donkey, it is a proof that the surface of the Earth is a plane surface, and, therefore, a proof that the Earth is not a globe.
“hundred miles below the sun and moon, [then it] cannot, by any known possibility come between them. It cannot therefore intercept the light of the sun, and throw its own shadow upon the moon. If such a thing were a natural possibility, how could the moon continue to shine during the whole or any considerable part of the period of its passage through the dark shadow of the earth? Refraction, or what has been called “Earth light,” will not aid in the explanation; because the light of the moon is at such times “like the glowing heat of firer tinged with deep red.” “Reddish is not the word to express it, it was red–red hot.” “The reddish light made it, seem to be on fire.” “It looked like a fire smouldering in its ashes.” “Its tint was that of red-hot copper.” The sun light is of an entirely different colour to that of the eclipsed moon; and it is contrary to known optical principles to say that light when refracted or reflected, or both simultaneously, is thereby changed in colour. If a light of a given colour is seen through a great depth of a comparatively dense medium, as the sun is often seen in winter through the fog and vapour of the atmosphere, it appears of a different colour, and generally of such as that which the moon so often gives during a total eclipse; but a shadow cannot produce any such effect, as it is, in fact, not an entity at all, but simply the absence of light.
As we have seen that there is, really no south point (or pole) but an infinity of points forming, together, a vast circumference -- the boundary of the known world, with its battlements of icebergs which bid defiance to man's onward course, in a southerly direction - so there can be no east or west "points,' just as there is no "yesterday," and no "tomorrow." In fact, as there is one point that is fixed (the North), it is impossible for any other point to be fixed likewise. East and west are, therefore, merely directions at right angles with a north and south line: and as the south point of the compass shifts round to all parts of the circular boundary, (as it may be carried round the central North) so the directions east and west, crossing this line, continued to form a circle at any latitude. A westerly circumnavigation, is going around with the North Star continually on the right hand, and an easterly circumnavigation is performed only when the reverse condition of things is maintained, the North Star being on the left hand as the journey is made. These facts, taken together, form a beautiful proof that the Earth is not a globe.
If the Earth were a globe, people - except those on the top - would, certainly, have to be "fastened" to its surface by some means or other, whether by the "attraction" of astronomers or by some other undiscovered and undiscoverable process! But, as we know that we simply walk on its surface without any other aid than that which is necessary for locomotion on a plane, it follows that we have, herein, a conclusive proof that Earth is not a globe.

61.) It is plain that a theory of measurements without a measuring-rod is like a ship without a rudder; that a measure that is not fixed, not likely to be fixed, and never has been fixed, forms no measuring-rod at all; and that as modern theoretical astronomy depends upon the Sun's distance from the Earth as its measuring-rod, and the distance is not known, it is a system of measurements without a measuring-rod – a ship without a rudder. Now, since it is not difficult to foresee the dashing of this thing upon the rock on which Zetetic astronomy is founded, i

The most commonly accepted explanation of this is that the space agencies of the world are involved in a conspiracy faking space travel and exploration. This likely began during the Cold War's 'Space Race', in which the USSR and USA were obsessed with beating each other into space to the point that each faked their accomplishments in an attempt to keep pace with the other's supposed achievements. Since the end of the Cold War, however, the conspiracy is most likely motivated by greed rather than political gains, and using only some of their funding to continue to fake space travel saves a lot of money to embezzle for themselves.